The present disclosure relates to a developing device which converts by electrophotography an electrostatic latent image into a manifest image, and relates to an image forming apparatus equipped with the developing device.
A typical developing device transfers toner included in a developer containing portion from a first rotational body to a second rotational body, and then, supplies the toner from the second rotational body to a photosensitive member. In a developing device of this type, toner is transferred from the developer containing portion to the first rotational body, the toner is transferred from the first rotational body to the second rotational body, and then, the toner is transferred from the second rotational body to the photosensitive member. Such transfer of the toner is performed by generating a potential difference between the first rotational body and the second rotational body, and a potential difference between the second rotational body and an electrostatic latent image formed on the photosensitive member.
When the photosensitive member is exposed and an electrostatic latent image is formed on the exposed portion, an edge electric field is generated in the boundary part (hereinafter, referred to as edge part”) with a non-exposed portion in the exposed portion. The edge electric field is an electric field generated due to the potential difference between the exposed portion and the non-exposed portion, and is also referred to as insulation electric field. That is, in the edge part of the exposed portion, the edge electric field is generated in addition to an electric field generated due to the potential difference between the electrostatic latent image and the second rotational body. Thus, the strength of the electric field in the edge part is greater than that in the other part of the exposed portion. Thus, a phenomenon occurs in which the amount of toner adhering to the edge part of the exposed portion is greater than the amount of toner adhering to the part other than the edge part of the exposed portion. This phenomenon is referred to as edge phenomenon.
Here, in the case where a recording sheet on which to form an image in the image forming apparatus is, for example, thick paper, when a fixing process is performed by a fixing device, a large amount of heat is taken by the thick paper compared with that in the case of an ordinary recording sheet. In this case, there is a risk of causing insufficient fixation of the toner to the thick paper. Thus, in a typical image forming apparatus, a series of image forming speeds (hereinafter, also simply referred to as image forming speed) including the conveying speed of the thick paper and the developing speed by the developing device are set to be slower than those in the case where image forming is performed on a recording sheet having an ordinary thickness. Accordingly, a sufficient amount of heat is provided to the thick paper during the fixing process.
Also when the image forming mode is a mode in which an image is formed at a high resolution, there are cases where the image forming speed is set to be slower than that in the case where image forming is performed on a recording sheet at an ordinary resolution.
If the image forming speed is set to be slower than the speed in ordinary time, the time period in which the toner can be transferred to the edge part of the exposed portion is extended, and the amount of toner adhering to the edge part of the exposed portion due to the electric field near the boundary is further increased. Thus, the edge phenomenon appears in a noticeable manner, which can cause a problem of reduced image quality, such as a line image becoming thicker than that in image data.